1. Field of the Invention
The present invention relates to a color processing apparatus and a color processing method for converting an input color into an output color within an output color gamut.
2. Description of the Related Art
With recent popularization of personal computers and workstations, desktop publishing (DTP) and computer-aided design (CAD) have come into wide use. Under these circumstances, a color reproduction technology for matching a color displayed on a monitor by a computer with a color reproduced on an output medium has gained in importance. For example, in DTP, a user carries out work based on a flow of creating, editing, and processing a color image on the monitor and outputting a resultant image via a color printer. In this workflow, the user has a strong desire for perceptual matching between the color image on the monitor and the printer output image.
However, in the case of the color reproduction technology, achievement of perceptual matching between the color image on the monitor and the printer output image is difficult. This difficulty arises for the following reason.
In a color monitor, light of a predetermined wavelength is emitted from a fluorescent material to display a color image. On the other hand, in a color printer, light of a predetermined wavelength is absorbed by ink, and a color image is represented by residual reflected light. Such a difference in image display form causes a large difference in color gamut between the color monitor and the color printer. Even in the case of color monitors, color gamuts vary between a liquid crystal monitor and a cathode-ray tube monitor of the electron gun type. Even in the case of color printers, color gamuts vary due to a difference in paper quality or ink composition. Thus, with regard to an image on a color monitor and a color printer output image, or color printer output images output from several types of machines or with plural types of paper qualities, complete matching of colors of these images is difficult in terms of colorimetry.
For such display media different from each other in color gamut, as a technology for absorbing the color gamut difference and perceptually matching display images with each other, a gamut mapping technology for mapping a certain color gamut into another color gamut in a L*a*b* space or a JCh space is available. The following gamut mapping technologies have conventionally been known.    (1) A method for mapping the entire color gamut linearly.    (2) A method for mapping only colors outside a color gamut to the surface of the color gamut.    (3) A method for preserving the original color as much as possible for in-gamut colors, and mapping out of gamut colors to high saturated region of the gamut, for example toward a point on the L axis, to preserve gradation.However, according to such a conventional gamut mapping technology, optimal mapping is carried out for each printer color gamut. Thus, when the same image is output with printers of different color gamuts, colors of the output image may be different.
Because of a difference in color gamut, apparently, reproduction of a completely identical color cannot be realized. Advisably, however, the color appearance of an output image has to be similar as much as possible.
Japanese Patent Application Laid-Open No. 2004-64668 discusses a method for increasing color identity even when output conditions are different by correcting a target color in an equi-hue direction according to the output condition of a printer.
The method discussed in Japanese Patent Application Laid-Open No. 2004-64668 can increase color identity with respect to an output color gamut. However, as target color data is designed optimal for an input color gamut, if an input color gamut varies, target color data has to be changed for each input color gamut. Accordingly, plural sets of target colors are required. Preparation of a target color beforehand for an optional input color gamut is difficult. As a result, highly accurate color reproduction irrespective of an input color gamut is difficult to be realized.